DISPLAY DEVICE OF THE FLAT-PANEL TYPE COMPRISING AN ELECTRON TRANSPORT DUCT AND A SEGMENTED FILAMENT

Abstract: A display device comprising electron-transport ducts having entrance apertures for electrons and means for generating electrons and injecting electrons into the transport ducts via said entrance apertures. Measures have been taken to improve the picture quality. For example, the location of the entrance aperture is not the same for each transport duct, but instead shows a variation. This enables an improved control of the electron flow in the transport ducts to be achieved; in particular crosstalk between the transport ducts can be reduced.

Description: Display device of the flat-panel type comprising an electron transport duct and a segmented filament. The invention relates to a display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen. A display device of the type mentioned in the opening paragraph is described in EP-A 0 436 997. In EP-A 0 436 997 a description is given of a display device in which the means for generating and injecting electrons comprise a line cathode which generates electrons which are injected into the transport ducts by means of electrodes. In general, the aim is to obtain display devices which are constructed so that, in operation, the electron flows in the transport ducts do not deviate from a desired value or deviate so little that it is not disturbing or only to a very small degree. Variations in electron flows, both in one transport duct and between transport ducts, cause a deterioration of the picture quality. It is an object of the invention to provide a display device of the type described in the opening paragraph, in which variations in the electron flows are precluded or reduced. Within the scope of the invention, it has been recognized that the cooperation between and mutual influencing of the means for generating electrons and injecting electrons into the transport ducts are of essential importance in this respect. This recognition is elaborated in various aspects of the invention. According to a first aspect of the invention, the display device is characterized in that the means for generating electrons comprise juxtaposed line cathodes. If one line cathode is used as the source for all transport ducts in the display device, problems with the homogeneity of the image displayed are, for example, caused by a relatively great voltage drop across the line cathode (of the order of magnitude of, for example 100 volts). As a result thereof, the effectiveness with which electrons are generated by the line cathode and injected into the transport ducts varies along the line cathode. Due to this, differences between electron flows in different transport ducts occur. The homogeneity of the image displayed is adversely affected thereby. A long line cathode is also sensitive to vibrations. Vibrations cause changes in the position of the line cathode relative to the transport ducts, which may result in variations in the electron flows and hence a reduction of the homogeneity of the image displayed. By using a number of line cathodes, the voltage difference across each line cathode and the vibration sensitivity of each of the line cathode can be separately reduced. This results in an improved homogeneity of the image displayed and hence an improved picture display. In an embodiment of the display device in accordance with the first aspect of the invention, the display device comprises a mixing means which is located between the line cathodes and the transport ducts and which serves to mix electron flows generated by the line cathodes before the electrons are injected into the transport ducts. This results in an improved homogeneity of the image displayed. Said mixing means causes the electrons generated by various line cathodes to be mixed before the electrons are injected into the transport ducts. When the electrons are mixed, a number of transport ducts receive electrons originating from more than one line cathode. By virtue thereof, electron-emission differences between line cathodes are less visible. In addition, adverse effects caused by differences in the positions of the line cathodes relative to the transport ducts are reduced. Intensity differences caused by the fact that the line cathodes are discrete electron sources which are separated from each other are also reduced. In a further embodiment of the display device in accordance with the first aspect of the invention, said display device comprises a first means for extracting electrons from the line cathodes via apertures which allow passage of electrons, and a transport surface which extends between said apertures and the transport ducts and which serves to transport electrons to the transport ducts by means of secondary emission and to mix electron flows. The use of a transport surface enables a proper mixing of electrons to be achieved which are transported by means of secondary emission via said transport surface. In an embodiment of the display device in accordance with the first aspect of the invention, the display device comprises at least two juxtaposed rows of line cathodes and the mixing means comprises one or more apertures for each row of line cathodes to allow passage of electrons and said mixing means comprises a means for joining electrons after they have passed through rows of apertures and directing them towards a row of entrance apertures of the transport ducts. In this embodiment, the intensity and homogeneity of the electron flows can be increased. Each entrance aperture of a transport duct receives electrons originating from at least line cathodes pertaining to different rows. Embodiments of the display device in accordance with a second aspect of the invention are characterized in that each transport duct has an entrance aperture for electrons and in that the location of the entrance aperture relative to the transport duct varies. As mentioned hereinabove, transport ducts in which the electron transport takes place by means of secondary emission of electrons are known from European Patent Application EP-A 0 436 997. In said Patent Application EP-A 0 436 997, a description is given of a display device in which each transport duct has an entrance aperture which is identically located relative to the transport duct. One aspect of the invention is based on the insight that, as regards the location of the entrance apertures, it is advantageous to so construct the transport ducts that they are not identical, i.e. the location of the entrance apertures varies. In the display device in accordance with the state of the art, each transport duct has an entrance aperture which is identically located relative to the transport duct. As a result, the entrance apertures are arranged in a row. By so positioning the entrance apertures a signal or electron flow in a transport duct can adversely affect the electron flow in an adjacent transport duct in a relatively simple manner. This phenomenon, hereinafter also referred to as "crosstalk", causes undesired variations in the electron flows. This phenomenon can be reduced by varying the location of the entrance apertures. This enables the distance between the entrance apertures to be increased and hence crosstalk to be reduced. Preferably, entrance apertures of adjacent transport ducts are situated at different locations. This results in a substantial reduction of crosstalk. In a further embodiment entrance apertures of adjacent transport ducts have different dimensions. By virtue thereof, systematic deviations of the cut-off voltage or of the emission of electrons into the apertures can be corrected. Preferably, juxtaposed transport ducts have a different line cathode as the source of electrons. If juxtaposed transport ducts have the same line cathode as the source of electrons, a difference in emission between line cathodes may bring about disturbing light and dark bands in the image. If juxtaposed transport ducts have different line cathodes as the electron source, this phenomenon is less likely to occur and is less disturbing. In display devices in accordance with a third aspect of the invention, electrodes are arranged around the entrance apertures and electrodes of adjacent apertures are separated from each other by electrically conductive and earthed surfaces. By virtue thereof, "crosstalk" caused by electrodes influencing each other is reduced. Also in such embodiments, preferably, the location of the entrance apertures varies and differs from that of adjacent entrance apertures. In this manner, more space is created for the conductive surfaces which has a favourable effect on the reduction of crosstalk. Display devices in accordance with a fourth aspect of the invention are characterized in that the means for generating electrons and injecting electrons comprise an emission source and, for each channel, a control electrode and an anode which are situated on the entrance side and exit side, respectively, of an entrance aperture of a transport duct, and in that the display device comprises means for applying electric potentials to the control electrode and anode to control the electron flow in the transport duct, said control electrode and anode being provided on surfaces of an electrically insulating plate whose thickness is maximally twice the smallest dimension of the entrance aperture. Preferably, the voltage on the anode is low. If the thickness of the plate is more than twice the smallest dimension of the entrance aperture, the voltage to be applied to the anode must be so high (for example < 300 V) that electron transport in the transport duct is adversely affected. Display devices in accordance with a fifth aspect of the invention are characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has an entrance aperture which is elongated, for example rectangular, and oriented so as to extend parallel to the line cathode. Vibrations in the line cathode affect the position of the line cathode relative to the entrance aperture. Such vibrations cause the number of electrons injected into the transport duct to vary. This variation leads to a deterioration of the quality of the image displayed. This effect is smaller for a rectangular entrance aperture than for, for example, a circular entrance aperture. Display devices in accordance with a sixth aspect of the invention are characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has an entrance aperture, said entrance aperture being tapered in form, with the largest aperture facing the line cathode. By virtue thereof, a higher percentage of electrons emitted by the line cathode is injected into the transport duct and the sensitivity to vibrations of the line cathode is reduced. Display devices in accordance with a seventh aspect of the invention are characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has a control electrode and an anode, and in that the display devices comprise means for applying electric potentials to the control electrode and the anode to control the electron flow, said line cathode extending between the control electrode and the anode. Such a construction will hereinafter also be referred to as inverted triode, since the sequence of the cathode (K), control electrode (Gl) and anode (G2), which is normally K-G1-G2, is inverted to G1-K-G2. If use is made of an inverted triode, either a high gain or a low gain can be chosen. If a high gain is chosen, it is possible to use relatively low voltages and a simple construction, if a low gain is chosen more favourable tolerances are obtained. The construction preferably comprises an ion trap. Aspects of the invention can be combined. However, this is not necessary, although some combinations offer further advantages. These and yet other aspects of the invention will be explained in greater detail by means of the following exemplary embodiments in which corresponding components generally bear the same reference numerals. Figs. IA and IB are a partly perspective view and a cross-sectional view, respectively, of a flat display device. Figs. 2A and 2B are sectional views of a detail of a flat display device in accordance with the invention. Figs. 3A up to and including 3C graphically show the mixing of electron flows. Figs. 4A and 4B show a detail of a further example of a flat display device in accordance with the invention. Figs. 5A and 5B show a detail of a further example of a flat display device in accordance with the invention. Figs. 6A up to and including 7B show a detail of a display device, respectively, in a cross-sectional view of a transport duct (Figs. 6A and 7A) and viewed in a direction transverse to a line cathode (Figs. 6B and 7B). Figs. 7C up to and including 7F show further aspects of embodiments of the invention. , Figs. 8 up to and including 19 show details of further embodiments of the display device in accordance with the invention. The drawings are diagrammatic and are generally not drawn to scale. In general, identical parts bear the same reference numerals. Figs. IA and IB show a display device 1 of the flat-panel type in a partly perspective view and in a sectional view, respectively. Display device 1 has a transparent front wall (window) 3 and a rear wall 4 which is located opposite said front wall. An electroluminescent screen 7 is provided on said window. Transport ducts 11 for transporting electrons extend parallel to the rear wall in, for example, the y-direction. In this example, the electrons are transported through a transport duct by applying a potential difference across the transport duct. The display device further comprises a means for extracting electrons from the transport ducts 11 at previously determined locations. In this example, this means is formed by apertures 8 in cooperation with electrodes 9 on walls 10. The display device further comprises means for directing electrons towards the luminescent screen. In this example, said means comprise electrodes 9 and the luminescent screen 12, between which a potential difference is applied. The display device comprises a means for injecting electrons into the transport ducts 11. In this example, said means comprises a system of cathodes 5 and electrodes G^ and G 2 . Gi is a control electrode which, in this example, can be driven separately for each individual channel. G 2 is an electrode which is common to several channels. The line cathode and the G l and G 2 electrodes together form a triode. Electrons are injected into the transport ducts by heating the cathodes 5 and applying a potential difference between the cathodes 5 and electrode G 2 . The display device further comprises control electrodes G j for controlling the intensity of the electron flows in the transport duct. The display device described in EP-A 0 436 997 comprises one line cathode which forms a plurality, for example 600, of electron sources for the transport ducts by means of electrodes G l and G 2 . The disadvantage of the use of one line cathode is that in a display device comprising a relatively long line cathode (for example in excess of 0.10 meter) problems regarding the homogeneity of the displayed image occur. A difference in homogeneity shows itself in differences in intensity and/or colour of the image displayed. It is an object of the invention to provide a display device in which the above problem is alleviated. , When a long line cathode is used, problems with regard to the homogeneity of the image displayed are caused, for example, by a relatively large voltage difference across the line cathode (of the order of magnitude of for example 100 volts). Due to this, at a constant voltage on G j and/or G 2 , the effectiveness with which electrons are emitted by the line cathode and injected into the transport ducts varies along the line cathode, so that the electron flow in a transport duct is influenced and hence the homogeneity of the image displayed adversely affected. The voltage difference across the wire is not constant but depends on, inter alia, the temperature of the line cathode and is subject to ageing. Besides, a long line cathode is sensitive to vibrations. Vibrations cause changes in the position of the line cathode relative to the transport ducts, which may also lead to a reduction of the homogeneity of the image displayed. By using a number of line cathodes, the voltage difference across each of the line cathodes and the sensitivity to vibrations of the individual cathodes can be reduced. This results in an improved homogeneity of the image displayed. Figs. 2 are sectional views, respectively, in an x-y plane (Fig. 2A) of the line cathodes and in an y-z plane, of a display device according to the invention. The display device comprises a row of line cathodes 20. In operation, said line cathodes are heated and electrons are injected into space 23 by means of electrode 22 (G^. Electrode 22 has apertures to allow passage of electrons. A potential difference is applied between electrode 22 and electrode 24. Preferably, a means for mixing electron flows before they are injected into the transport ducts is present between said electrodes. Said mixing means comprises a transport surface 25. Secondary emission initiates an electron flow from electrode 22 to electrode 24 via transport surface 25. For a description of electron transport by means of secondary emission, reference is made to EP-A 0 436 977. Viewed in a direction from the apertures in electrode 22 to the apertures in electrode 24 the intensity differences decrease substantially. Figs. 3A up to and including 3C illustrate this phenomenon. Fig. 3A shows, viewed in a direction parallel to the line cathodes, the electric current directly behind the apertures in electrode 23; the display device comprising three line cathodes. Fig. 3B shows the electric current halfway between electrodes 23 and 24, and Fig. 3C shows the electric current directly in front of electrode 24. It is clearly visible that transport surface 25 smoothens the differences between the electric currents. This has a positive effect on the image displayed. Figs. 4A and 4B show a further example of the display device in accordance with the invention. In this example, the display device comprises two rows of line cathodes 41 and 42. Electrons 45 emitted by the line cathodes are injected into the transport ducts 11 by electrode 43 which comprises one aperture 44 for each transport duct. The line cathodes and the apertures 44 are arranged so that a substantially equal number of electrons passes through each aperture 44. In this example, the emitted electrons are thus directly injected into the transport ducts from the line cathodes without the electron flows being mixed. A disadvantage of this arrangement is that a small difference in the position of the line cathodes 41 and/or 42 relative to the apertures 43 may cause relatively large intensity differences between the electron flows in the transport ducts. Preferably, the electron flows are mixed to improve the homogeneity of the image displayed. Figs. 2A and 2B, as described above, show an example of a display device in which the electron flows are mixed. Figs. 5A and 5B show a further example of the display device in accordance with the invention. In this example the display device comprises two rows of line cathodes 51 and 52. Electrons emitted by the line cathodes 51 and 52 are injected into space 55 by electrodes 53 and 54. Electrodes 53 and 54 have a number of apertures to allow passage of electrons. In space 55 the electron flows of both rows of line cathodes are mixed by means of secondary emission via walls 56 and directed towards apertures 57. This aspect of the invention is not limited to the above-described examples. It is noted that this aspect of the invention relates to the part of the display device which serves to generate electrons and inject them into the transport ducts. In the examples, the transport of the electrons in the transport ducts takes place by means of secondary emission. As regards this aspect of the invention, electron transport may alternatively take place, for example, via electron beams. In Figs. 2A, 2B, 4A and 4B, the electrodes 22 (Figs. 2A and 2B) and 43, 44 (Figs. 4A and 4B) have a large number of apertures. The invention is not limited thereto. The electrodes 22, 44 and 45 may alternatively be provided with a slit-shaped aperture extending over the length of the row(s) of line cathodes or with more than one slit-shaped aperture. The line cathodes may be situated in many different ways with respect to the transport ducts, for example directly underneath the transport ducts (see for example Figs. IA and IB), directly above the transport ducts, on the upper side of the transport ducts and next to the transport ducts (see for example Figs. 2A and 2B) on the underside of the transport ducts and next to the transport ducts (see for example Figs. 4 A and 4B), next to the transport ducts and between the upper side and the underside of the transport ducts (see for example Figs. 5A and 5B). The display device may alternatively comprise more than one system of transport ducts, for example a first system of transport ducts extending from the upper side of the display device halfway down the display device, with, for example, on the upper side of the display device a row of sources cooperating with the first system of transport ducts; and a second system of transport ducts extending from the underside of the display device halfway up the display device, a second row of sources which is located on the underside of the display device cooperating with the second system of transport ducts. The number of electrons which can be extracted from a transport duct is governed by the number of electrons which is injected into the transport duct. Problems which arise during injecting electrons into the transport ducts are illustrated in Figs. 6A and 6B. Fig. 6A shows, viewed in a direction transverse to a line cathode 61 and entrance apertures 62, a line cathode and the entrance apertures. Fig. 6B shows, in a cross-sectional view of a transport duct 11, the location of the entrance apertures 62. In said Figures, the location of the entrance apertures of each transport duct is identical. As the transport ducts are arranged so as to form a row, the entrance apertures also form a row. In this example, the display device comprises one long line cathode as the emission source for all apertures. The entrance apertures 62 are surrounded by electrodes 64. Voltages on said electrodes control the number of electrons which are injected into an entrance aperture. The number of electrons will hereinafter also be referred to as "the electron flow in the transport duct" or "the electron flow" for short. The accuracy of the electron flows in the different transport ducts and hence the quality of the image are adversely affected by a number of effects. Part of the electrons intended for aperture 62A will go to aperture 62B, and an electric voltage on electrode 64A induces a disturbing voltage on electrode 64B. This disturbing voltage may unintentionally affect the number of electrons injected into the transport duct corresponding with entrance aperture 62B. These phenomena will hereinafter also be referred to as "crosstalk" between two transport ducts. Crosstalk reduces the quality of the image displayed, more particularly the contrast of the image displayed. One aspect of the invention is based on the insight that crosstalk can be reduced by a variation in the location of the entrance apertures. This is shown in Figs. 7A and 7B. Fig. 7A shows, transverse to the line cathodes and the entrance apertures, a detail of a display device comprising two line cathodes 71 and 72 and entrance apertures 73 and 74. Fig. 7B shows a cross-section of a transport duct. In Fig. 7B both entrance apertures of the relevant transport ducts are shown; in the display device each transport duct 11A, 11B, 11C etc. , has one entrance aperture. As in the transport duct the electrons are transported over surfaces by means of secondary emission, the location of the entrance apertures relative to the transport duct and, more particularly, relative to the means for extracting electrons from the transport ducts has no influence or a negligible influence on the quantity of electrons extracted from the transport duct. Within the framework of the invention it has been recognized that this aspect is a characteristic of such transport ducts. In systems where an electron beam is transported by means of electron-optical electric fields, the entrance aperture is electron-optically imaged on an exit aperture. In such transport ducts, a change in the location of the entrance aperture causes the electron optical image to change and hence is of great consequence. By varying the location of the entrance aperture, the distance between the entrance apertures 73 and 74 is increased without increasing the distance between the transport ducts, as a comparison between Figs. 6A and 7A shows. As a result thereof, crosstalk between transport ducts 11 A, 11B etc. is reduced, so that an improved picture quality can be attained. Figs. 6A up to and including 7B show embodiments of a display device in which the means for generating electrons comprise a line cathode. Also if for each entrance aperture one separate emission source, for example a punctiform cathode, is used, the aspects of the invention shown in Figs. 6A up to and including 7B, which are not related to disturbing effects of vibrations of a line cathode, are useful to preclude crosstalk between transport ducts. However, within the scope of the invention, the use of at least one line cathode which serves as the emission source for more than one transport duct is preferred. In comparison with a display device in which a separate emission source is used for each transport duct, this enables the number of electron sources to be reduced, allowing, in general, a simpler and better control of the electron flows in the transport ducts and resulting in fewer problems regarding display devices which are rejected during the manufacture and/or the homogeneity of the image displayed. The disadvantages of the use of only one line cathode have been system forth hereinabove. The problems connected with these disadvantages can be reduced by employing a number of line cathodes as described above. Figs. 6A up to and including 7B also show further aspects of the invention. The G and G 2 electrodes are situated on the entrance side and the exit side, respectively, of the entrance apertures. Both electrodes are provided on an electrically insulating plate. The thickness d_l of said plate is less than twice the smallest dimension h_lb. If the thickness is more than twice said dimension, the voltage to be applied to electrode G 2 is so high that the injection of electrons into the transport duct and the transport of electrons in the transport duct are hampered. In this example, d_l is 550 μm and h_lb is 800 μm. The entrance aperture is conically shaped, with the largest aperture facing the line cathode. By virtue thereof, the percentage of electrons emitted by the line cathode which is injected into the transport duct is increased and the sensitivity to vibrations of the line cathode is reduced. The entrance apertures are rectangular (or square) and extend parallel to the line cathode. In comparison with, for example, circular entrance apertures this results in a reduced sensitivity to vibrations of the line cathode. Fig. 7A further shows that an electrically conductive layer 75 which is connected to earth is provided between the electrodes G j . This reduces the electrical crosstalk between the different electrodes G j . The conductive layer 75 can alternatively be connected to a fixed electric potential. Fig. 7C is a top view of the electrode G2 which is common to a number of apertures. In the vicinity of said electrode G2 there is also provided an electrically conductive layer 76 which is earthed or connected to a fixed electric potential. The Gl electrodes are control electrodes which modulate the emission of electrons from the wire cathode by means of a negative accelerating voltage V ol . An electrode G2 which produces a positive accelerating voltage V o , is situated behind electrode Gl. Apertures corresponding to a transport duct are provided in both electrodes. The emission is modulated per channel and each channel has an individual Gl electrode. The electrode G2 is common to a number of channels. The electrode Gl is driven by high-frequency video signals. A great voltage swing to modulate the emission is undesirable. The current-voltage characteristic is optimal when the so-called cut-off voltage (= the voltage at zero emission) is low and the slope is steep. Fig. 7D compares the current-voltage characteristic of round apertures, 1.2 mm across, in Gl (solid-line curve) to the current-voltage characteristic of rectangular apertures having dimensions of 1.2 mm x 0.5 mm, the longest dimension being situated along the wire cathode (broken- line curve). The voltage on the Gl electrode is plotted on the horizontal axis (V 01 ), the emission through the hole in the Gl electrode (Ie) is plotted on the vertical axis. It is clearly visible that rectangular apertures require a lower (absolute) voltage on the Gl electrode and that the slope of the curve is steeper. Figs. 7E and 7F show the effect of "crosstalk" between adjacent channels. The voltage on the Gl electrode (V 01 ) is plotted on the horizontal axis, and the emission through the hole in the Gl electrode (Ie) is plotted on the vertical axis. Fig. 7E depicts a situation in which all apertures are arranged in a row (an example thereof is shown in Fig. 6A). Preferably, the emission into an aperture in a Gl electrode is not or hardly governed by the voltage on an adjacent Gl electrode. In the opposite case, the contrast of the image displayed is reduced. Fig. 7E shows two curves, a solid-line curve and a broken-line curve. The solid-line curve represents the emission through an aperture in a Gl electrode when an electric potential of -10 V is applied to both adjacent Gl electrodes, the broken-line curve represents the emission when an electric potential of -20 V is applied to both adjacent Gl electrodes. It is clearly visible that the magnitude of the potential on adjacent Gl electrodes has a considerable effect on the emission. Fig. 7F depicts the emission (vertical axis) through an aperture in a Gl electrode as a function of the electric potential on the Gl electrode (horizontal axis), the apertures in the Gl electrode being staggered relative to each other. The solid-line curve and the broken- line curve substantially coincide. The on-off characteristic is much better defined now. Influencing the emission into one channel by means of the voltage on an adjacent Gl electrode may for example have the effect that horizontal lines have another intensity than vertical lines. The reason for this is that if a horizontal line (i.e. extending transversely to the direction of the transport ducts) is displayed, the voltage on a number of juxtaposed Gl electrodes is identical, namely approximately of the order of 0 V, whereas during displaying a vertical line (i.e. along a transport duct) one Gl electrode is at a voltage of approximately 0 V, while the adjacent Gl electrodes all are at a voltage of approximately -10 V or lower. In addition to the above-mentioned effect (the possible dependence of the emission of electrons into an aperture of a Gl electrode on the electric voltages on adjacent Gl electrodes) there is a second effect. It has been found that when the apertures in the Gl electrodes are arranged in a row (as shown in Fig. 6 A) the required cut-off voltage is higher than when the apertures are staggered relative to each other (as shown in, for example, Fig. 7A). As mentioned hereinabove, the cut-off voltage is preferably as small as possible. Figs. 8 up to and including 14 show a number of possible further embodiments comprising a number of line cathodes. In Fig. 8 use is made of a number of juxtaposed line cathodes 81. The length of the line cathodes is small and microphonism is reduced. Of each line t cathode only the central portion (between the hatched portions) is used as an emission source. Entrance apertures 82 of transport ducts 83 are only located opposite said central portion. In said portion the temperature variation is small. Fig. 9 shows a combination of the embodiments of Figs. 7 and 8. In these and subsequent Figures (with the exception of Fig. 10) juxtaposed transport ducts always have a different line cathode as the source for electrons. This is a preferred embodiment. If juxtaposed transport ducts have the same line cathode as the source for electrons, emission differences between line cathodes may bring about disturbing light and dark bands in the image. If juxtaposed transport ducts have different line cathodes as the electron source, this phenomenon is less likely to occur and is less disturbing. Fig. 10 shows a number of obliquely juxtaposed line cathodes 101. Fig. 11 shows a variant thereof. In said Figures, the vertical lines represent the transport ducts, the dots represent the entrance apertures of the transport ducts and the lines interconnecting said dots represent the line cathodes. Fig. 12 shows a further variant. In the examples shown above, the entrance apertures are located in one plane. Fig. 12 is an embodiment in which three cross-sections of adjacent transport ducts comprising entrance apertures which are situated in different planes are shown. By virtue thereof, the distance between apertures can be further increased and hence crosstalk reduced. The entrance aperture 121 for the transport duct 122 is situated in side wall 123. For adjacent transport ducts the entrance aperture is situated in side walls 124 or 125. Line cathode 120 is also shown. Fig. 13 shows a further variant. The transport ducts 130 comprise entrance apertures in wall 131 or 132 or 133. For each system of entrance apertures the display device comprises a line cathode 134, 135 or 136, as shown in Fig. 13. It will be obvious that within the scope of the invention many variations are possible. For example, Fig. 14 shows an arrangement in which line cathodes 141 are used which are arranged in a zigzag way relative to the entrance apertures 142. Fig. 15 shows yet another embodiment of the display device in accordance with the invention. This embodiment is characterized in that closely spaced apertures for allowing passage of the electrons have different dimensions. Apertures 150 and 151 in the adjacent Gl electrodes have different dimensions. The wire cathodes 155 are supported by supporting elements 156. Said supporting elements influence the electric fields in the vicinity of the apertures 151 and hence the emission from the wire cathodes 155 into the apertures 151. This causes a deviation of the cut-off voltage for the apertures 151 relative to the apertures 150. This systematic deviation is corrected in this exemplary embodiment by selecting the dimensions of the holes 151 so that they are different from the dimensions of the holes 150. In this example the apertures 151 are slightly larger than the apertures 150. This example gives one possible embodiment. In another embodiment, for example, the size of the apertures is a function of the distance between the aperture. and the beginning or end of the wire cathode. In operation, a temperature difference may occur along the wire cathode. This results in a difference in emission along the wire cathode, such that in general the emission is greatest halfway between the supporting points and decreases towards said supporting points. This systematic variation in emission can be corrected by varying the size of the apertures in such a manner that the size of the apertures increases from the centre of the wire cathode towards the supporting points. Fig. 6 shows a construction in which the sequence of the cathode (K), electrode (Gl) and electrode (G2) is K-G1-G2. Within the framework of the invention it has been recognized that a number of improvements can be made in this construction. Figures 16 and 17 show constructions of the triode part in which the line cathode 161 extends between the control electrode Gl and the anode G2. Consequently, the sequence has been changed to Gl- K-G2. This enables a better control of the emission of the line cathode. Fig. 16 shows an embodiment in which the Gl electrode is arranged very closely around the line cathode. This arrangement provides a high gain, i.e. a higher current is supplied at a lower modulation voltage, as compared to the construction shown in Fig. 6. Fig. 17 shows an embodiment in which the electrode Gl is situated at a greater distance from the line cathode. This arrangement has a relatively low gain. Both an increase and a decrease of the gain may be advantageous. A high gain reduces the proximity effects (crosstalk). A low gain has the advantage of a more stable operation and less slump. Slump is a phenomenon which is characterized by a reduction of the emission of the cathode after some time. Table 1 shows some of the advantages of the constructions shown in Figs. 16 and 17. For the Gl electrodes use can be made of a tinplate construction or of a metal film on a support 181, for example a glass support. A construction of the type in which the Gl electrode comprises a ridge in which the cathode extends, as shown in Fig. 16, is a preferred construction because it enables the influence of the control electrode on the emission of the line cathode to be accurately controlled and a high gain to be attained. In particular for this construction it is advantageous if the Gl electrode extends on the support. In that case, the dimensions of the Gl electrode can be accurately controlled. Fig. 18 shows a possible construction. The Gl electrodes are in the form of a metal film in a groove 162, for example ground or sandblasted in a glass element. This can be carried out very accurately and, hence, has important advantages for the mechanical tolerances. Fig. 19 is a sectional view of a preferred embodiment of an inverted triode arrangement (i.e. an arrangement G1-K-G2). This preferred embodiment comprises an ion trap 191. To this end, the transport duct 192 comprises an electrode 193 to which a voltage is applied such that positive ions are attracted. In this Figure, the positive ions are indicated by dotted lines. The electron paths are indicated by full lines. In each of the constructions shown in Figs. 6, 16 and 17 such an ion trap can be realized in the entrance portion of the transport ducts. However, such an ion trap can particularly suitably be used in inverted triode arrangements. Fig. 19 further shows a number of electrodes 194, each electrode being at a higher positive voltage than the preceding electrode to cause the electron transport in the transport duct to take place in the direction indicated by the arrows. At the other end there is arranged an electrode 193 which captures the ions formed in the channel in the vicinity of the triode. The influence of the ion trap on the emitted electrons is small due to the high velocity of the electrons relative to the velocity of the ions. Such an ion trap protects the wire cathode K against sputtering. In this example, the ion trap is situated in a blind portion of the transport duct. Summarizing, it may be said that within the scope of the invention it has been recognized that the cooperation between and mutual influencing of the means for generating electrons and injecting electrons into the transport ducts are of fundamental importance to the functioning of the display device. Said recognition has been elaborated in the different embodiments of the invention shown herein. Further it has been recognized within the scope of the invention that, unlike customary display devices having an electroluminescent screen, in which an electron-optical image is produced from an electron source on a spot on the screen, in a display device in accordance with the invention, in which electrons are transported via transport ducts, an electron-optical image, in which the electron beam is imaged by means of lenses as though it were a light beam, is not produced. In the display device in accordance with the invention, the manner in which the electrons are transported rather resembles a sluice system. This insight opens up many possibilities, including the use of mixing means (practically impossible with electron-optical images), a variation of the position of the entrance aperture relative to the rest of the transport duct (very difficult, if not practically impossible with electron-optical images), a rectangular entrance aperture (very difficult to image electron-optically), a tapered entrance aperture (ditto) and an inverted triode.

Claims: Claims: 1. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that the means for generating electrons comprise juxtaposed line cathodes. 2. A display device as claimed in Claim 1, characterized in that the display device comprises a mixing means which is located between the line cathodes and the transport ducts and which serves to mix electron flows generated by the line cathodes before the electrons are injected into the transport ducts. 3. A display device as claimed in Claim 2, characterized in that the display device comprises a means for extracting electrons from the line cathodes via apertures which allow passage of electrons, and a transport surface which extends between said apertures and the transport ducts and which serves to transport electrons to the transport ducts by means of secondary emission and to mix electron flows. 4. A display device as claimed in Claim 3, characterized in that the display device comprises at least two juxtaposed rows of line cathodes, and in that the mixing means comprises one or more apertures for each row of line cathodes to allow passage of electrons, and in that said mixing means comprises a means for joining the electrons after they have passed through rows of apertures and directing them towards a row of entrance apertures of the transport ducts. 5. A display device as claimed in Claim 1, 2, 3, 4 or 5, characterized in that each transport duct has an entrance aperture for electrons, and in that the location of the entrance aperture relative to the transport duct varies. 6. A display device as claimed in Claim 5, characterized in that entrance apertures of adjacent transport ducts are situated at different locations. 7. A display device as claimed in any one of the preceding Claims, characterized in that juxtaposed transport ducts have a different line cathode as the source of electrons. 8. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that each transport duct has an entrance aperture for electrons and in that the location of the entrance aperture relative to the transport duct varies. 9. A display device as claimed in Claim 8, characterized in that entrance apertures of adjacent transport ducts are situated at different locations. 10. A display device as claimed in Claim 8, characterized in that entrance apertures of adjacent transport ducts have different dimensions. 11. A display device as claimed in Claim 8 or 9, characterized in that the entrance apertures are not located in one plane. 12. A display device as claimed in Claim 8, 9, 10 or 11, characterized in that the means for generating electrons comprise at least one line cathode which serves as the emission source for more than one transport duct. 13. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that each transport duct has an entrance aperture for electrons, in that electrodes are arranged around the entrance apertures, and in that electrodes of adjacent apertures are separated from each other by electrically conductive and earthed surfaces. 14. A display device as claimed in Claim 13, characterized in that the location of the entrance aperture relative to the transport duct varies. 15. A display device as claimed in Claim 14, characterized in that entrance apertures of adjacent transport ducts are situated at different locations. 16. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that the means for generating electrons and injecting electrons comprise an emission source and, for each channel, a control electrode and an anode which are situated on the entrance side and exit side, respectively, of an entrance aperture of a transport duct, and in that the display device comprises means for applying electric potentials to the control electrode and anode to control the electron flow in the transport duct, said control electrode and anode being provided on surfaces of an electrically insulating plate whose thickness is maximally twice the smallest dimension of the entrance aperture. 17. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has an entrance aperture which is rectangular and oriented so as to extend parallel to the line cathode. 18. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a row of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, means for applying potential differences across the transport ducts to transport electrons through said transport ducts by means of secondary emission, means for extracting electrons from the transport ducts and means for directing electrons towards the electroluminescent screen, characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has an entrance aperture which flares towards the line cathode. 19. A display device having a vacuum envelope, said display device comprising an electroluminescent screen and a number of transport ducts for transporting electrons, means for generating electrons and injecting electrons into the transport ducts, characterized in that the means for generating electrons and injecting electrons comprise a line cathode and each transport duct has a control electrode and an anode, and in that the display device comprises means for applying electric potentials to the control electrode and the anode to control the electron flow, said line cathode extending between the control electrode and the anode. 20. A display device as claimed in Claim 19, characterized in that an electrode which is connected to earth extends between the control electrode and the cathode. 21. A display device as claimed in Claim 19 or 20, characterized in that the control electrode comprises a ridge in which the line cathode extends. 22. A display device as claimed in Claim 21, characterized in that the control electrode is situated on a support. 23. A display device as claimed in any one of the preceding claims, characterized in that an ion trap is situated in a portion of the transport duct, and a negative voltage can be applied to said electrode to capture ions.

Inventors: BALLER THEUNIS SIEMEN, COSMAN EDWARD CHRISTIAAN, DE ZWART SIEBE TJERK, LAMBERT NICOLAAS, MONTIE EDWIN ANDRE, RADEMAKERS ANTONIUS JOHANNES J, TROMPENAARS PETRUS HUBERTUS FR, BALLER, THEUNIS, SIEMEN, COSMAN, EDWARD, CHRISTIAAN, DE ZWART, SIEBE, TJERK, LAMBERT, NICOLAAS, MONTIE, EDWIN, ANDRE, RADEMAKERS, ANTONIUS, JOHANNES, JOSEPHUS, TROMPENAARS, PETRUS, HUBERTUS, FRANCISCUS
Applicants: BALLER THEUNIS SIEMEN, COSMAN EDWARD CHRISTIAAN, LAMBERT NICOLAAS, MONTIE EDWIN ANDRE, PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, RADEMAKERS ANTONIUS JOHANNES J, TROMPENAARS PETRUS HUBERTUS FR, ZWART SIEBE TJERK DE, BALLER, THEUNIS, SIEMEN, COSMAN, EDWARD, CHRISTIAAN, DE ZWART, SIEBE, TJERK, LAMBERT, NICOLAAS, MONTIE, EDWIN, ANDRE, PHILIPS ELECTRONICS N.V., RADEMAKERS, ANTONIUS, JOHANNES, JOSEPHUS, TROMPENAARS, PETRUS, HUBERTUS, FRANCISCUS
Application year: 1994
Main classification: H01J31
IPC codes: H01J31